(i) Field of the Invention
The present invention relates to a method for obtaining high-purity cinnamic acid, and more particularly, it relates to a method which can be applied to a crude cinnamic acid derivative-containing mixture formed by reacting a styrene, an alcohol, carbon monoxide and oxygen in the presence of a metallic catalyst in an organic solvent to obtain cinnamic acid having a high purity of 98% or more and containing less metal impurities.
(ii) Description of the Related Art
Cinnamic acid be obtained in the form of its ester from styrene or its derivative, carbon monoxide, an alcohol and oxygen.
As methods for purifying crude cinnamic acid esters, there are a distillation process and a crystallization process which are usual purification methods, but a part of the cinnamic acid ester is inevitably lost in the purifying operation, so that after hydrolysis, the yield of cinnamic acid deteriorates. Additionally, for the purification, expensive distillation facilities and crystallizing/filtering facilities are required.
Even if such a crude cinnamic acid ester is directly subjected to a conventional hydrolysis treatment, cinnamic acid having a high purity and containing less metal impurities cannot be industrially obtained.
Heretofore, various methods for the synthesis of cinnamic acid have been known, but a method has been suggested which comprises reacting styrene or its derivative, carbon monoxide, an alcohol and oxygen in the presence of a catalyst. According to this method, a cinnamic acid ester can be obtained, but a heavy metal which is used as the catalyst is taken into the resultant reaction mixture. Most of the heavy metal is removed from the reaction mixture by filtration, but 10 to 80% of the heavy metal component remains dissolved in the reaction mixture containing the cinnamic acid ester as the main component. In addition, unreacted styrene and by-products are also dissolved in the reaction mixture.
In order to obtain a high-purity cinnamic acid from such a reaction mixture in which the heavy metal, styrene and the by-products are dissolved, a method has been heretofore used which comprises distilling and purifying the reaction mixture to form a high-purity cinnamic acid ester, and then hydrolyzing the formed ester to obtain cinnamic acid. In particular, for the complete removal of the heavy metal from the reaction mixture, the distillation is essential.
However, in the practice of the distillation, expensive distillation facilities are necessary, and tar is formed during the distillation and consequently, some loss of the cinnamic acid ester is unavoidable.
The present inventors have intensively investigated the hydrolysis of the reaction mixture containing the cinnamic acid ester. As a result, they have found that when the crude cinnamic acid ester is directly hydrolyzed without purifying the cinnamic acid ester and the combination of an active carbon treatment and extraction with an organic solvent is then carried out, the high-purity cinnamic acid can be obtained in a high yield. In consequence, the present invention has been completed. Heretofore, the active carbon treatment is effective for the removal of organic compounds and a gas, but there has not been known a method for completely removing the heavy metal by the active carbon treatment, as in the present invention.